Sound recording system



Feb. 22, 1938.

E. s. GILLE ET AL 2,14%,815

SOUND RECORDING SYSTEM Filed July 16, 1934 a Sheets-Sheet '1 Feb. 22, 1938. E 5 g E -r AL 2Jea5 SOUND RECORDING SYTEM Filed July 16, 1934 V 3 Sheets-Sheet 2 v K 50 w ?d 71 W H w M r Mi k ,X

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Feb. 22, 1938.

E. s. GILLE ET AL SOUND RECORDING SYSTEM Filed July 16, 1934 3 Sheets-Sheet 3 Patented Feb. 22, 1938 UNITED STATES PATENT OFFICE Calif.,

assignors of one-half to Robert W.

Spcfford, Los Angeles, Calif.

Application July 16, 1934, Serial No. 735,415

16 Claims.

Our invention relates to the art of recording and reproducing sounds or other phenomena by the use of a light-sensitive element such as a motion-picture film. While the invention will 5 be described with reference to a variable-area sound-recording system, it should be understood that we are not limited thereto, for many of the principles utilized in the present invention can be applied to a variable-density recording system.

Commercial sound-recording systems now in general use form a sound record including both the high frequency and the low frequency undulations to be recorded. If the resultant sound record is of the variable-area type, the sound track carries opaque portions in the form of relatively broad crests representing the lower frequencies, the higher frequencies being superimposed on these crests and appearing in the form of minute lines extending therefrom. We have a found that more satisfactory results can often 4 be obtained by forming in effect two sound records on the same sound track, and the present invention includes among its objects the provision of a novel method and apparatus for accomplishing this result.

We have also found that in many instances it is better to separately record the high frequency band and the low frequency band, or to form on one portion of the film a conventional sound record and to form on another portion of the film a sound record which supplements the first record and adds thereto any deficiencies in low or high frequency undulations, and it is an object of the present invention to provide a novel method and apparatus for thus separately recording bands of different frequency range usually on the same sound track.

However, the invention is not limited to the simultaneous formation of two records, for in m some instances these records can be placed on spaced portions of the sound track at different periods of time. In other instances these records can be formed from separate sources, such as separate microphones. It is an object of the present invention to provide a method and apparatus whereby such results can be accomplished.

Regardless of whether or not the undulations to be recorded on the two portions of the sound 59 track are of different frequency range, we have found it desirable to record these undulations by the use of a pair of light beams which usually move toward and away from each other in response to the undulations to be recorded, and

the provision of a novel system whereby this is made possible is among the important objects oi the present invention.

In such a system we find it preferable to use two mirrors for forming the beams by reflected light, and to separately drive these mirrors by 5 two armatures, and it is an object of the present invention to provide such a system. The advantage of such a system lies in the direct control of both armatures made possible by electrical or mechanical adjustments of the two light in, reflecting systems.

It is a further object of the invention to subject such armature structures to substantially equal quantities of magnetic flux and usually to connect the armatures in such relationship v; with respect to the incoming impulses that a given impulse will swing the mirrors in opposite directions.

Another object of the invention is to provide a recording system utilizing a beam of light, one edge of which comprises a control edge and extends obliquely across a light slit, this beam being moved in a direction non-parallel to said control edge whereby varying portions of the light slit are exposed to the beam. Such a system can be used to advantage in increasing the sensitivity of the device, and it is often possible to use two beams each providing such a control edge. If two beams are used, the control edges of these beams are usually formed by the adjacent portions of the beams.

Further objects of the invention include the provision of a novel relationship between the light source and one or more mirrors, and the provision of a novel light train. The invention also h includes a novel light train. The invention also includes a novel positioning of this light train with relation to one or more mirrors.

The provision of a new type of magnetic circuit for the armature is also included among the objects of the invention, as well as a bridge con- 40 struction of novel form and provided with novel damping means.

It is a further object of the invention to provide a mirror structure which is readily adjustable with respect to the light train.

Present recording and reproducing systems are open to the defect that due to grain structure of the film, extraneous vibration, etc. there are reproduced certain background noises even during the time that no sound is reaching the microphone of the recording system.

It is an object of the present invention to provide a novel method and apparatus for overcoming this defect by utilizing in conjunction with a light slit suitable means preventing light from passing through the light slit during the time that no sound is reaching the microphone, but allowing the light rays to pass through the light slit as soon as any sounds are transmitted to this microphone.

Further objects and advantages of the invention will be evident to those skilled in the art from the following description.

Referring to the drawings,

Fig. 1 is a diagrammatic view illustrating a simplified form of the invention.

Fig. 2 is a diagrammatic view of another form of the invention in which the light beam moves obliquely with respect to the light slit.

Fig. 3 is a fragmentary view of a film and illustrates diagrammatically one type of sound record which can be obtained from the system shown in Figs. 1 and 2.

Figs. 4, 5, and 6 represent alternative systems for energizing the armatures.

Fig. 7 is a side elevational view of the recording unit of the invention.

Fig. 8 is a top view of this recording unit.

Fig. 9 is a sectional View taken on the line 99 of Fig. 8.

Fig. 10 is a view of the control mechanism taken as indicated by the arrow is of Fig. 8.

Fig. 11 is a sectional view of the control mechanism taken on the line li-H of Fig. 10.

Fig. 12 is a horizontal sectional. view of this mechanism taken on the line l2-l2 of Fig. 10.

Fig. 13 is an enlarged view of a portion of Fig. 12.

Fig. 14 is a sectional view of an alternative form of the invention.

Fig. 15 is a sectional view taken on the line !5|5 of Fig. 14.

Figs. 16 and l? are diagrammatic views illustrating the principles used in the form of the invention shown in Fig. 2.

In general, the preferred embodiment of the invention directs light rays from a light source to a pair of mirrors, these mirrors reflecting a pair of beams toward the light-sensitive element, the quantum of light reaching the light-sensitive element being controlled in response to the undulations to be recorded. usually by the use of a drive means for the mirrors and a light slit disposed in the path of the reflected light rays.

Such a system is shown in l in which a light source it: directs light rays toward a pair of recording systems including mirrors it and ii. It is usually preferable, though not invariably necessary, to use an aperture plate it which, in conjunction with a condenser lens it, forms a pair of light beams which reach the mirrors !5 and II. This aperture plate preferably provides two apertures separated by a bar 26. While these apertures are shown as being bounded on all sides by portions of the aperture plate, this is not essential to the invention, the bar 28 being the important factor if an aperture plate is used. In the form shown in Fig. 1 we provide adjacent lenses 22 and 23 positioned in the respective beams reaching the mirrors i6 and i! and adapted to form identical images of the light source on these mirrors.

The reflected beams may pass through lenses 24 and 25 and impinge on any suitable means forming a light slit 2%. This means is diagrammatically shown in Fig. 1 as including a plate Z'i in which the light slit 26 is cut, the ends of this light slit being bounded by cut-oii edges 28 and 253.

As best shown in Fig. 1, the reflected beams respectively cover the two outer portions of the light slit 26. This light slit is relatively narrow and permits the passage therethrough of two planes of light respectively indicated by the numerals 3.5 and 3i. These planes of light move through a lens system 33 and are focused on a sound track 3 1 of a light-sensitive element 35 shown as being in the form of a motion picture film. The planes or light reach this sound track on opposite sides of a medial section thereof so that one plane of light exposes one portion of the sound track and the other plane of light exposes the other portion of the sound track. If desired the cut-off edges 28 and 29 may be so disposed that they cut off any light rays which might otherwise tend to reach the film 35 at a section outside of the sound track 3 3. In other instances auxiliary masking means can be utilized to accomplish this result in which event the position of the cut-off edges 28 and 29 is not critical, it being often possible to dispense with these edges and use two spaced plates to form the light slit 26.

In the embodiment of the invention shown, the reflected beams are caused to move toward and away from each other in response to the undulations to be recorded, this being accomplished by the use of a suitable control'means operatively connected to the mirrors i6 and II. This is preferably accomplished by mounting the mirrors on corresponding armatures 36 and 37 which, in the preferred embodiment of the invention, are in the form of loops or" wire strung across a suitable bridge structure as will be hereinafter described. These armatures are subjected to the same or substantially identical flux densities. In Fig. l we have shown these armatures as being mounted in the same magnetic field set up between pole pieces 35a and 37a, thus insuring that these armatures will be subjected to equal fields. The pole pieces Sta and 37a may be portions of a permanent magnet or portions of a coreof an electromagnet.

When this system is used for recording sound the sound undulations may be picked up by a microphone 33 and transmitted through a suitable amplifier 3567, to an input winding so of a transformer ill. The secondary of this transformer is indicated by the numeral ii and is center-tapped. One-half of this winding ii is electrically connected in circuit with the armature means 36 associated with the mirror I5, while the other half of this winding is electrically connected in circuit with the armature means 3'6 associated with the mirror El. Usually these armature means are connected in reversed relation to the winding 4% when the armatures are positioned in the same magnetic field so that a given current impulse reaching the winding -i will move the armatures 36 and 31 in opposite directions, thus causing the reflected beams to move toward and away from each other. As the reflected beams move toward and away from each other, the quantum of light reaching the sound track increases or decreases, for the planes of light 3G and iii will increase or decrease in width and the lineal images thus formed on the sound track will correspondingly increase or decrease in length.

If the armatures 36 and 31 and their associated mirrors l6 and I! are of identical construction, and if the same current impulses are delivered to each other, identical sound records will be formed on opposite sides of the sound track. Each of these records will appear, upon development, somewhat in the form of minute lines extending inward from the edges of the sound track, such a record being diagrammatically illustrated in Fig. 1. The resulting record can be developed and used for reproducing the original sound undulations in any desired manner, such for instance, as directing a beam of light simultaneously through both records on the sound track and either directly or indirectly into controlling relationship with a photo-electric cell. It will also be clear that the two sound records will not lose their identity even if prints are made from the light-sensitive element 35. In this instance the central portion of the sound track will be opaque on these prints, while the edges of the sound track will be transparent. While it is usually preferable in conventional reproducing structures to influence the photo-electric cell by light rays passing through both of the sound records at the same time, it is also possible to use these sound records to separately control the light reaching a pair of photo-electric cells, thus permitting separate reproduction of these records.

In many instances we find it desirable to operate one of the armatures at high frequency and the other at a lower frequency. The result will be the formation of a sound track such as diagrammatically illustrated in Fig. 3, a portion 45 thereof containing a record of the lower frequencies while a portion 46 contains a record of the higher frequencies present in the same sound undulations. This may be accomplished in several ways. In the first place, it is possible to design the armatures and their associated mirrors so that they are of different mass and responsive to different bands of frequency. Further, the armatures may be tuned to different frequencies by changing the tension in the loops of wire forming these armatures. A preferred method, however, is to use one or more filter means in conjunction with one or both of the armatures 3S and 31. Such a filter means may be in the form of a band-pass filter, or a filter which passes frequencies either above or below a given frequency. Thus, if desired, the complete range of frequency may be transmitted to the armature 36 and a suitable filter means 48 used in circuit with the armature 37 to filter therefrom a given band of frequency. This system can often be used to advantage to supply deficiencies in either the high or low frequencies in the original sound. Thus, if the sound to be recorded is deficient in low frequencies, due for instance to the bass section of an orchestra being too far from the microphone, or a human voice or chorus inherently defective in the amount of lower frequencies, or due to the inability of the recording system to properly respond to the low frequencies in the necessary degree, one sound record can be supplemented by a second sound record recording only the lower frequencies in which the first record is deficient. Similar remarks apply to a sound or sound record deficient in the desired amount of high frequencies. Such dual recording eliminates the present necessity for a projection operator in a theatre continuously adjusting or shifting the compensator or tone control when the sound record shifts from music to speech or vice versa.

On the other hand, it is sometimes preferable to use two filter means, supplementing the filter 48 by a filter 49 connected in circuit with the armature 36, one filter passing only high frequencies and the other passing only low frequencies. Such a filter system is diagrammatically shown in Fig. 1, these filters being connected across the conductors extending from a winding M. It will be clear, however, that these filters may be of any suitable design and may be connected in these circuits in series or series-parallel relationship, depending upon the design of the filter. Such filters ordinarily comprise properly designed impedances and condensers, and the design thereof is well known in the art.

In some instances it is possible to use combinations of the inertia, tension, and frequency control systems set forth above to permit one mirror to oscillate at a higher frequency than the other.

Various systems for delivering the electrical impulses to the armatures may be used. Alternative systems are diagrammatically illustrated in Figs. 4, 5, and 6, no attempt being made to show the filter systems therein. In the connections indicated in Fig. 4 the winding ll of the transformer 4?) is not center-tapped but is connected in parallel with the armatures 3S and 3'! in reversed relationship. In the form shown in Fig. a pair of output tubes 55 and 5! are connected in push-pull relationship. The plate circuit of one tube is connected to a conductor 52 through which current may flow to the armature 31, returning to the filament circuit through a suitable source of potential 53. The plate circuit of the tube 553 may be connected by a conductor 54 to one terminal of the armature 35, the other terminal being connected to the source of potential 53. It will be clear that any suitable filters can be used in conjunction with the circuit shown in Figs. 4 and 5 to permit oscillation of the mirrors at difierent frequency.

In the mode of connection illustrated in Fig. 6 a double-field system is used, as will be hereinafter described in detail, one armature being in each field. Here the armatures are shown as being connected in series across the secondary winding 4! of the transformer. Such a system has been found to give excellent fidelity of reproduction.

In applying these principles in a commercial structure, we have found it desirable to use a recording unit such as indicated in Figs. '7 to 13. Referring particularly to Fig. 8, the recording unit includes a base 51 to which an angled bracket 58 is attached, this bracket supporting the light source it. In this form of the invention the light train is included in tubes 59 and 60 which are disposed at an angle with respect to each other, the condenser lens 99, the aperture plate 18, and the lenses 22 and 23 being secured in the tube 58, while the lenses 2% and 25, the plate defining the light slit 2%, and the lens system 33 are disposed in the tube 60. The photosensitive element moves through a suitable guide 6! which provides an aperture 62 masking the light rays to confine these light rays to the sound track. With this system the side walls of the aperture 62 can be used in place of the cut-off edges 28 and 29 of the light slit to prevent light from reaching portions of the light-sensitive element outside the sound track.

The tubes 59 and 69 are rigidly connected together and are mounted above the base 51 by the use of a bracket 6%. Screws 65 secure this bracket to the base 5! and, if desired, may be of smaller diameter than the holes in the bracket 54 through which they extend so as to permit adjustment between the base 5? and the tubes 59 and This adjustment is not. however, es-

sential in all instances for the control mechanism indicated in general by the numeral I is mounted so as tobe readily adjustable relative to the elements of the light train retained in the tubes 59 and 80.

In effecting this adjustable mounting of the control mechanism 76 relative to the base 57 we prefer to secure a base plate TI to the base 51, this base plate including one or more grooves I2 in which tongues '33 of an intermediate plate 14 extend, thus permitting the plate It to move relative to the base plate II in a direction determined by the grooves '32. Adjustment of these plates may be effected by any suitable means such as the bracket F on the base plate ii and journalling a knurled screw IE which is threaded into the intermediate plate I4.

Above the intermediate plate I4 is another intermediate plate 18 which is slidable relative to the intermediate plate '14 in a direction transverse to the groove 72. This may be accomplished by providing an upwardly extending tongue 79 on the intermediate plate l4 and fitting into a corresponding groove of the intermediate plate '18. Adjustment between the plates is effected by any suitable means such as a bracket 80 on the intermediate plate i i and journalling a knurled screw 8! threaded into the intermediate plate 18.

Positioned above the intermediate plate '28 is a mounting plate 83 which is preferably pivoted with respect thereto by means of a pin 04 extending into aligned openings of the plates I8 and 83 (see Figs. 8 and 11). To control this pivoting movement any suitable adjusting means can be provided between the plates '58 and 33. As shown, this means includes a bracket 85 secured to the intermediate plate 18, a compression spring 36 positioned between the mounting plate 83 and this bracket. At the opposite end of the intermediate plate 18 is a bracket 8'! to which is threaded a knurled screw 88 which bears against the mounting plate 83. By turning this screw 88 the mounting plate 83 is pivoted relative to the intermediate plate 18, the spring 86 maintaining contact between the screw 88 and the mounting plate 83. The pivot pin 84 is preferably positioned so that the axis thereof if projected willextend between the mirrors i6 and i! so that the control mechanism is can be pivoted about an axis adjacent the mirrors.

The mounting plate 83 provides four upwardly extending arms 90 which support the magnetic core 9! of the invention in a manner best shown in Fig. 12, this core being retained between the legs $5 by any suitable means such as set-screws 92. These set-screws can be used for adjusting purposes, though this is not essential to the invention in view of the adequate adjustment offered by the supporting means for the mounting plate 83.

As best shown in Fig. 12, this core includes a rear-member 93 to which two side members 90 and '95 are connected either integrally or by the use of screws 95a. To the forward end of the side members 94 and 95 are adjustably secured pole pieces 96 and 9?. This adjustment may be effected in any one of a number of ways. As shown, screws 98 connect these pole pieces to the side members, and the screw-receiving holes of the pole pieces are somewhat larger than the screws so as to permit adjustment between the pole pieces and the side members. A central leg I00 extends forward from the rear member 93 and includes a tongue I0! which extends into the space between the adjacent ends of the pole pieces ,96 and 9'l'to define gaps I02 and I03.

Such a core structure in effect provides two flux paths, one path including the central leg we, the gap I03, the pole piece 91, and the side member 95, the other flux path including the central leg I00, the gap I02, the pole piece 96, and the side member 94. It is usually desirable to maintain an electromagnetic field of high fiux density in the gaps E02 and I03. This may be accomplished by the useof one or more magnetizing windings. Two such windings are shown, indicated bythe numerals I04 and I05, being respectively positioned to surround the side members es and 95. sities in the gaps I02 and I03 may be effected eitherby an adjustment of the magnetizing current flowing through these windings or by the change in the relative gap spacings as may be effected by the adjusting screws 98.

The armatures 36 and iii are mounted on a novel bridge construction best shown in Figs. and 11. This bridge construction includes a body I I {3 preferably formed of insulating material and providing upper and lower supporting members HI and H2. Preferably this body extends on opposite sides of the central leg I00, and the preferred embodiment completely surrounds this central leg, this being accomplished by the use of an opening H3 in the body H0 which snugly engages the central leg I00. Any suitable means may be used to clamp the body with respect to this'central leg, or the body may be frictionally retained in place. This structure permits the upper and lower supporting members III and I I2 to be respectively positioned immediately above and below the gaps.

Equalization of the flux den- Each of the armaturesfsfi and 31 is in the form r of a loop, the ends of this loop being respectively connected to terminals H5 and H0 mounted in a terminal plate III formed of insulating material and secured to the posts 99. Essentially this loop includes two conducting members I I 8 and H9 electrically connected to complete the loop, though other types of armatures can be utilized without departing from the spirit of the invention. The loop formed by the conducting members H8 and H9 preferably extends over a roller I20 pivoted between bifurcations of an arm I2I which is in turn pivoted to the body H0 as by a pin I 22. The rear end of the arm IZI is shown as being connected by a tension spring 424 to an arm I25 which is in turn pivoted at I25 to the body- II 0. Adjustment of the arm I25 is made possible by the use of a screw I27 extending therethrough and being threadedly received by the body H0. The tension in the conducting members H3 and II!) can thus be changed by turning the screw I21, the spring I 2-5 acting to prevent the application of excessive tension, and

permitting a resilient adjustment otherwise'impossible. If desired the-conducting members H8 and H9 can be anchored to the bottom of the body IIO, being respectively connected by auxiliary leads to the terminals I I5 and I I6.

As will be apparent from Fig. 11, the lower supporting member IIZ-provides a supporting surface for the conducting members H8 and II9 which is curved with a large radius of curvature. The upward supporting member III is more sharply curved and contacts the conducting members H8 and II 9 over only a small area. The lower supporting member II2 thus provides a large surface of contact for the conducting members I I8 and I I 9 which not only holds these conducting members in properly spaced relationship but through friction supports these conducting members so that tension can be applied. The smaller surface of contact between the supporting member III and these conducting members permits sliding of these conducting members to effect this change in tension. If desired, these supporting members may be slightly grooved to hold the conducting members in properly spaced relationship.

The mirror It can be connected to the conducting members H8 and H9 by any suitable means. We find it preferable to use conducting members in the form of ribbons to permit more effective attachment of the mirror to these members. Our method of connecting the mirror to these conducting members is, we believe, novel, and in explaining this connection, reference should be had to the greatly enlarged sectional view illustrated in Fig. 13 which is purposely distorted to show the details of construction. In mounting the mirror IE to the conducting members H8 and H9, for instance, we prefer to first apply tension to these members after they are connected to the bridge structure, and then to apply a small quantity of adhesive material to the small space between the conducting members H8 and H9 in such a position as to contact the rear face of the mirror I6 which is temporarily held in place manually or by suitable clamping means. After this adhesive material has hardened we apply a coating of adhesive material to the backs and sides of the conducting members H8 and H9 at the section adjacent the mirror and extending forward along the sides of the mirror I6, as shown in Fig. 13. This firmly holds the mirror in place. In addition, we have found it very desirable to damp theconducting members by coating these members with the adhesive so that the adhesive extends on all sides thereof to form an envelope therearound after the adhesive material has hardened. We have found it preferable to thus coat the conducting members H8 and H9 not only between the supporting members III and H2 but also above the supporting member III. The taut portions of the conducting members I I8 and H9 between the supporting member III and the roller I20 have been found to introduce distortion into the recording system if not damped. We have found that a coating of adhesive material to these portions will overcome this distortion. However, there is no necessity for coating the lower portions of the conducting members H8 and H9 which contact the supporting member H2. So also, it is not always essential to coat that portion of the conducting members H8 and I I9 which actually engages the supporting member I I I, thus permitting a freer movement of these members relative to the upper supporting member H I. Various liquid adhesive substances, which upon hardening will form a durable coating, may be used, a mixture of shellac and varnish having been found to be very satisfactory.

It will be understood that the armature 36 may be similarly formed so as to be separately adjustable and so as to be similarly damped, this armature including conducting members extending through the gap I02.

It will be clear that the adjusting means for the control mechanism permits the shifting of the mirrors relative to the light train to properly direct the light rays toward the light-sensitive element 35. Thus, the mirrors may be moved toward and away from the tubes 59 and 60 by the use of the knurled screw '56, or can be moved transversely with respect to these tubes by the knurled screw 8!. The angular relationship between the mirrors and these tubes can be changed by adjustment of the knurled screw 88.

An additional feature of the invention which is often extremely desirable, but which is not essential to the operativeness of the system, includes a means of preventing at least a portion of the light rays from passing through the light slit during the time that no sound reaches the microphone 38. As best shown in Figs. 1 and 9, this means may take the form of a bafiie or paddle which, if desired, may have a straight upper edge extending parallel to the light slit, as shown in Fig. 2, and be of such size as to cover the essential portions of this slit when moved into an upper position. Preferably, however, this upper edge is curved as best shown in Fig. 9 having a central crest I 3| and troughs I32 and I 33. In Fig. 9 the normal inactive positions of the beams of light are indicated by dotted lines I3 1 and I35, and the bafile or paddle I3!) is shown in its upper position. It will thus be apparent that at this time the troughs I32 and I33 allow portions of the beams to pass through the light slit 26 though these portions will not necessarily extend to expose the extreme sides of the sound track. The crest I3I will prevent any stray light reaching the central portion of the track. When, however, it is desired to record sounds the baffle I30 will be moved downward a distance so that the crest I3I shields a smaller portion of the central portion of the sound track and the troughs permit more of the beams to impinge on the v structure forming the light slit. The angled sides of the troughs, together with the fact that the film is moving, preclude any sudden change of position of the opaque portion of the resulting sound record as might result in any audible click in the reproducing system. The baffle can be lowered so that the crest I3I clears the slot, but it is usually preferable to lower this bafile a distance which varies with the volume of the sound to be recorded, it not being necessary to have the crest I3I clear the slot unless the beams move almost into contact with each other.

Such retracting movement of the paddle may be made to take place against the action of one or more small springs I M and may be caused by any suitable means such as the energization of an electromagnet M2. The circuit can be so designed that current will flow through this electromagnet only during the time that sounds are reaching the microphone 38 or it may be designed or balanced so that the increase in current during this time will move the paddle I30 downward. In Fig. 1 we have diagrammatically shown the electromagnet I42 as being connected to an auxiliary secondary winding I43 of the transformer 46 through a variable resistance I45, In some instances it is desirable to prevent any tendency for the electromagnet I 42 to vibrate the bafiie I 35 upon the application of alternating potentials thereto. In such case a rectifier I41 can be connected in the circuit either with or without auxiliary chokes to permit rectification of the alternating currents and to further stabilize the baffle. Such a system allows the bafile to be lowered in degree determined by the volume or intensity of the sound to be recorded. So also, it is usually preferable to have a minute time delay between the operation of the baffle and the mirrors. This can be accomplished by proper control of the impedance of the recording circuits relative to the circuit including the electromagnet M2 to permit the baflle I30 to be moved downward an instant before the mirrors are shifted. Various modes of connecting the electromagnet Hi2 with relation to the circuit connecting the microphone and the armatures may be used, and will be readily apparent tothose skilled in the art.

In the form of the invention shown in Fig. 1 the incident light rays extend substantially horizontally toward the mirrors, the reflected beams also being substantially horizontal. In many instances we find it desirable to change the angular relationship of these light rays both from the standpoint of space considerations and from other standpoints to be hereinafter mentioned. A system for eifecting these results is shown in Figs. I l and 15.

In this form of the invention the tube 59 extends upward, while the tube 60 remains substantially horizontal. The incident light rays move downward toward the mirrors and are reflected substantially horizontally toward the photo-sensitive element 35. The bar 20 separating the apertures in the plate It extends in the plane of the paper as viewed in Fig. 14. The light train in this form of the invention is also somewhat modified, the lenses 22 and 23 being dispensed with. So also the lenses 2'3 and 25 are replaced by a lens system I59 through which the reflected light rays from each mirror pass. The light slit 26 is so mounted that it extends substantially transverse to the direction of movement of the photo-sensitive element 35.

. This form of the invention requires a somewhat different mounting structure for the control mechanism it, it being necessary to mount the mirrors in an inclined position. Such a structure is shown in Fig. 14 and includes an inclined block Hill disposed between the intermediate plate Hi and the intermediate plate 18, previously described. This block carries a groove receiving the tongue 19 of the intermediate plate l4, and is provided with a groove l6! on the sloping portion of the block which receives a tongue i622 extending downward from the intermediate plate 78. Any suitable adjusting means, not shown, may be provided for changing the position of the intermediate plate 18 relative to the inclined block E60, thus permitting movement of the mirrors in an inclined direction relative to the tubes 59 and 6D.

In this form of the invention we have found it advantageous to use a light source mounted on a bracket 55 and providing a lineal filament indicated by the numeral Hi8, and to position this lineal filament toextend transversely with respect to the mirrors and the apertures of the aperture plate iii, if used. Preferably the lineal filament 565 extendssubstantially parallel to a horizontal line joining the centers of the two mirrors. Such a positioning of the filament is very desirable, for the image of the lineal filament is thus thrown across the mirrors, the most intense light traversing the two apertures. An equal illumination of the mirrors by the most intense portion of the light is thus obtained.

In the forms hereinbefore described the refiected beams are moved toward and away from each other in a plane parallel. to the light slit. While this system is quite satisfactory in most installations, we have found that greater sensitivity can be obtained if these beams are moved toward and away from each other in a direction non-parallel to this light slit. Nor is this system limited in uitilty to the use of two beams, for we believe it to be novel to use a single beam with one edge extending obliquely with respect to the light slit, the beam being moved in a direction non-parallel to this edge.

These principles are best illustrated in Figs. 16 and 17. In Fig. 16 a single light beam, indicated by the dotted lines l 78, is shown. The axis of the light slit is indicated by the line CC, and, with the beam disposed as shown, the lineal image formed on the light-sensitive element at this instant of time will be of a length proportional to the distance B. If, however, the beam of light indicated by the numeral llt'moves in a direction indicated by the double arrow ill, it will be clear that even a slight movement of the beam in this direction will result in a much greater change in the length of the lineal image produced. Stated in other words, if we term the edge iii: of the beam i'lll as a control edge, this edge will extend obliquely relative to the light slit, and movement of the beam H in any direction non-parallel to this control edge I12 will change the length of the lineal image formed on the light-sensitive element. Preferably, we move the beam i'lil in a direction transverse to the control edge H2, though this is not always essential. It will be clear that this control edge may be determined by the bar 29 of the aperture plate 58 or may be determined by one edge of the mirro-r which forms the beam indicated by the dotted line Hi]. This increase in sensitivity will be obtained in the formation of the sound record regardless of whether or not two mirrors. are utilized or regardless of whether or not the beam H is formed by reflection from a mirror or is moved by other means, and this feature of the invention is thus not limited to the particular means shown for deflecting the light beam.

In Fig. 17 the same principles are shown as applied to a double-mirror system. Here the light beams are indicated by the dotted lines I and N5, the control edges being respectively indicated by the numerals I12 and H6, both control edges extending obliquely across the light slit 25. If these beams are moved toward and away from each other in a direction indicated by'the double arrow l 18, the lineal images formed'on each side of the medial section of the sound track will correspondingly change in length if identical impulses are delivered to the two armatures. In effecting movement of the beams in a direction indicated by the double arrow H8, it'is only necessary to turn the recording unit relative to the light slit so that the pivotal axes of the mirrors are parallel to the control edges H2 or I76. The mirrors are diagrammatically shown in such a position in Fig. 2. With such a system it is desirable to utilize a lineal filament in the light source and extend this filament substantially parallel to the mirrors.

It will be clear that with a system such as shown in Figs. 2, l6, and 17, the sensitivity of the recording structure will be materially increased, and that it is necessary to move the mirrors through materially smaller angles than would otherwise be the case.

It will be clear that various modifications can be made without departing from the spirit of the invention. Many of these variations have been set forth in detail hereinbefore, and others will be apparent to those' skilled in the art. It will also be clear that certain of the features herein shown and described find utility in other recording systems, and that certain of these features can be used with a single-mirror system rather than a double-mirror system.

We claim as our invention:

1. In a device for modulating quantum of light reaching a light-sensitive surface from a light source: a bridge structure including spaced support members and a pair of conducting members extending in taut relationship therebetween; a mirror secured to said conducting members; means for forcing flux across said conducting members; means for delivering to said conductors electric impulses to oscillate said mirror and damping means for said mirror and conducting members including a covering for each of said conductors and surrounding that portion of each of said conductors which extends in taut relationship between said support members.

2. A combination as defined in claim 1 in which said covering is the form of a hardened coating of adhesive liquid applied to the periphery of each conducting member.

3. In a device for modulating quantum of light reaching a light-sensitive element from a light source: walls forming a light slit; means forming a pair of light beams directed toward said light slit to impinge thereon at positions on opposite sides of the mid-section of said slit whereby the end portions of said light slit pass two lineal planes of light to said light-sensitive element at a given instant of time, the adjacent edges of said light beams forming control edges extending at acute angles relative to the longi tudinal axis of said light slit; and means for moving said beams toward and away from each other to change the length of the corresponding lineal planes reaching said light-sensitive element.

4. In combination in a system for recording sound on a track of a light-sensitive surface: means for directing opposed and narrow beams of light into exposing relationship with spaced sections of said sound track, said means including a pair of independently movable mirrors, a pair of armatures operatively and respectively connected to said mirrors, and means for sending magnetic flux across said armatures; means for impressing on one of said armatures electric undulations varying in response to the complete range of sound frequencies desired to be recorded to form a first sound record on one section of said light-sensitive surface, said one of said armatures and its associated mirror being incapable of uniformly recording all frequencies within said complete range whereby said first sound record is defective in certain frequencies; a filter means electrically connected to said last named means for filtering the electric undulations varying with said complete range of sound frequencies to produce a band of frequencies within said complete range, which band includes said certain frequencies in which said first sound record is defective; and means for transmitting said band of frequencies to the other of said armatures to vibrate the other of said mirrors at frequencies within this thereby forming a second sound record on said other section of said sound track which supplies the deficiences of said first sound record.

5. In combination in a system for recording sound on a sound tracl: of a light-sensitive surface: means for directing opposed and narrow beams of light into exposing relationship with spaced sections of said sound track, said means including a pair of independently movable mirrors and separate electric drive means for oscillating said mirrors; means for impressing on one of said drive means electric undulations varying in response to the complete range of sound frequencies desired to be recorded to form a first sound record on one section of said light-sensitive surface, said one of said drive means and its associated mirror being incapable of uniformly recording all frequencies within said complete range whereby said first sound record is defective in certain frequencies; a filter m ans electrically connected to said last named means for filtering the electric undulations varying with said complete range of sound frequencies to produce a band of frequencies within said complete range, which band includes said certain frequencies in which said first sound record is defective; and means for transmitting said band of frequencies to the other of said drive means to vibrate the other of said mirrors at frequencies within this band, thereby forming a second sound record on said other section of said sound track which supplies the deficiencies of said first sound record.

6. In combination: a core of magnetic material providing pcle pieces spaced from each other and a central leg extending into the space between said pole pieces but spaced from these pole pieces define two said core thereby providing two flux paths, one flux path including said central leg and one of said gaps and one of said pole pieces, and the other flux path including said central leg and the other of said gaps and the other of said pole pieces; means for sending magnetic flux through each of said paths; an armature in each of said gaps; and a mirror connected to each armature.

7. A combination as defined in claim 6, including means for adjustably mounting said pole ces to permit adjusting movement thereof toward and away from said central leg to adjust the relative dimensions of said gaps.

8. In combination: a core of magnetic material providing pole pieces spaced from each other and a central leg extending into the space between said pole pieces but spaced from these pole pieces to define two gaps, said core thereby providing two flux paths, one flux path including said central leg and one of said gaps and one of said pole pieces, and the other flux path including said central leg and the other of said gaps and the other of said pole pieces; a bridge structure spanning said gaps; an armature supported in each of said gaps by said bridge structure; a mirror connected to each of said armatures; and means for sending magnetic flux through each of said paths.

9. In a device for modulating a quantum of light reaching a surface from a light source; a mask having walls forming a light slit; means forming a light beam and directing said beam to impinge directly on said mask to pass a portion of said beam through said slit whereby a lineal plane of light reaches said surface, said beam having a control edge extending at an acute angle relative to the longitudinal axis of said slit; and means for moving said beam of light longitudinally of said slit to vary the length of said lineal plane of light reaching said surface.

10. In a light modulating device of the character described, the combination of: a bridge structure including spaced support members and a torsion member extending between said support members; a mirror secured to said torsion member; means for operating said torsion memher to oscillate said mirror; and adjustable support means for said bridge structure including means for moving said bridge structure in a plane parallel to the plane of said mirror, means for moving said bridge in a plane oblique to the plane of said mirror, and means for pivoting said bridge structure on an axis parallel to the plane of said mirror.

11. In a light modulating device of the character described, the combination of a bridge structure including spaced support members and a pair of conducting members extending in taut relationship therebetween; a mirror secured to said conducting members to be oscillated thereby when electric impulses are delivered thereto; and adjustable support means for said bridge structure including means for moving said bridge structure in a plane parallel to the plane of said mirror, means for moving said bridge structure in a plane oblique to the plane of said mirror, and means for pivoting said bridge structure on an axis parallel to the plane of said mirror.

12. In a light modulating device of the character described, the combination of: a bridge structure including spaced support members and a pair of conducting members extending in taut relationship therebetween; a mirror secured to said conducting members to be oscillated thereby when electric impulses are delivered thereto; and an adjustable support means for said bridge structure operable to mcve said mirror in three directions one of which is parallel to the normal plane of said mirror.

13. In a light modulating device of the character described, the combination of a bridge structure including spaced support members and a pair of conducting members extending in taut relationship therebetween; a mirror secured to said conducting members to be oscillated thereby when electric current is delivered thereto; and

tension means for adjustably varying the tension 7 40 of said conducting members.

14. In combination: a core of magnetic material providing pole pieces spaced from each other and a central leg extending into the space between said pole pieces but spaced from these pole pieces to define two gaps, said core thereby providing two flux paths, one flux path including said central leg and one of said gaps and one of said pole pieces, and the other flux path including said central leg and the other of said gaps and the other of said pole pieces; a bridge structure spanning said gaps; an armature supported in each of said gaps by said bridge structure; a mirror connected to each of said armatures to be oscillated thereby; and adjustable support means for said bridge structure operable to move said mirrors in three directions one of which is parallel to the normal planes of said mirrors.

15. In combination: a core of magnetic material providing pole pieces spaced from each other and a central leg extending into the space between said pole pieces but spaced from these pole pieces to define two gaps, said core thereby pro viding two flux paths, one flux path including said central leg and one of said gaps and one of said pole :pieces, and the other flux path including said central leg and the other of said gaps and the other of said pole pieces; an armature in each of said gaps; a mirror connected to each armature to be oscillated thereby; and adjustable support means for said armatures operable to move said armatures in three directions one of which is parallel to the normal planes of said mirrors.

16. A combination as defined in claim 15 including means for adjustably mounting said pole pieces to permit adjusting movement thereof toward and away from said; central leg to adjust the relative dimensions of said gaps.

EARL s. GILLE. BENSON D. GILLEL 

